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I am looking at a superheat charging chart and there is one thing I don't understand. I know how to use the chart, I just want to understand the thermodynamics.

When the ambient condensor temperature is constant, the system superheat increases with increasing return air temperature. OK, that one's obvious.

Now going through the chart in the other direction, for a constant return air temperature, the superheat DECREASES with INCREASING ambient condensor temperature.
So, if a system is charged correctly, the superheat will be SMALLER at a higher ambient condensor temp (for the same return air temp)?
Can someone explain the physics behind this? I don't understand why that is the case.

Originally posted by dpatty As the ambiant temp goes up so does the head pressure meaning it can push more liquid through the orifice. Hope that helps

I understand that the head pressure goes up with temperature, but it is still a closed system, so the density of the refrigerant that the compressor is pumping is still the same. So how can the compressor pump more liquid though the system? Your reply basically states that the mass flow of refrigerant is increased at higher mabient condensor temps, right? I don't understand why that happens.

Actually it's a combination of what I said and the other post. As pressure rises more liquid is able to get through the orifice, But since it is not rejecting as much heat you do not get an increase in performance and that causes the evap saturation temp to rise and unable to carry as much heat.

Ahhh.... I see now, so with increasing ambient condensor temps, the head pressure raises and obviously the liquid temp, then the evap temp and pressure is also higher, which causes less heat transfer to the refrigerant (at constant return air temp) which is at a higher pressure (so higher sat temp), so the suction temp and suction saturation temp get closer together...

It just seems so counterintuitive, but you have to keep in ind that superheat is just a delta temp. In this case, there's less heat gain for the refrigerant in the evap AND at the same time the pressure (and therefore sat temp) increases. So even though the actual temps have increased, the delta is decreased...

Originally posted by samtn but it is still a closed system, so the density of the refrigerant that the compressor is pumping is still the same. So how can the compressor pump more liquid though the system?

The vapor that is being pumped pushes the liquid.In other words,I think the liquid density is always the same.It doesn't change.

The vapor density does change,and it changes based on heatload and working pressures.

Originally posted by jacob perkins
The vapor that is being pumped pushes the liquid.In other words,I think the liquid density is always the same.It doesn't change.

The vapor density does change,and it changes based on heatload and working pressures.

[Edited by jacob perkins on 07-16-2005 at 12:50 AM] [/B]

Yes, the vapor pushes the liquid. But if the volume taken up by the liquid in the system is constant (with ambient temp), which should be the case more or less, since the liquid line is supposed to be completely in liquid form, and then there is a short section of the condensing coil which is liquid. Then the volume of the rest of the system where the refrigerant is in gaseous form is also constant. That means that the average density of the gaseous refrigerant is also constant. The only thing that could change with (ambient) temperature is that the density upstream of the compressor could become lower and the density downstream of the compressor then becomes higher. This in turn would mean that the compressor has a changing pressure ratio, which I find hard to believe for a positive displacement pump.

Bottomline: I still think that the decreasing superheat with increasing ambinet condensor temp is caused by the increase in pressure (not density, remember this is a closed system) that in turn increases the saturation temp. So it's the increasing saturation temp that is causing the superheat to decrease, not the lowering of the gas temp.

You might find this interesting,but I find it confusing and apt to give me a headache.However,I will try respond.

First,I will just state some facts that you seem to have missed.You stated something about liquid and vapor levels being constant.I dont know what you meant,but I can say with certainty that liquid and vapors are not constant but a dynamic in the system.The only constants I can think of are the weight of the refrigerant charge and the volume of all the interconnected componants.

let me tell you a little story...and since the purpose of mechanical refrigeration is cooling,our story begins in the evaporator:

At start up the heat load on the evaporator is high.Our evaporator will require alot of liquid refrigerant from the condenser ,to boil and absorb heat from the air passing over the coil.Our saturation temperture and superheat will be high,and our vapor will be very dense.This heavy suction vapor will require more work from our compressor.With an increased amp draw the compressor will send this vapor on to the condenser,where pressure will be increased to send more liquid back to where it is needed--the highly loaded evaporator.We will have less liquid in our condenser at this time,because conditions of the evaporator.

Now later that very same day...when the room temperture has dropped,the heat load on the evaporator is less.With lower evaporator heat load,our suction pressure,superheat,and vapor density levels have dropped.
The vapor enters the compressor,and very little electromechanical work is required to send it on to the condenser.This light vapor causes a lower highside pressure and more liquid being stored in the condenser circuit,and less being sent to the evaporator.

The moral of the story is this:The evaporator heat load and the liquid requirementof the evaporator are factors influencing the density of suction vapors.And suction vapors will influence the work of the compressor,highside pressure,and level of liquid storage in the condenser.

Or in other words,the way it was taught to me:
"the evaporator is the Boss and all other componants do what it says via its bully messenger,the vapor density.

And...it's saturday nite!

Sometimes there are compounding complexities of multiple variables that are not intuitively obvious